This invention relates to sheets of brazing alloy foil, clad sheets of aluminium for use in the brazing of large section structures, brazed large section aluminium structures or methods of brazing large aluminium structures and is essentially concerned with the brazing alloys for such products and processes.
In the brazing of aluminium and its alloys one difficulty which is encountered is the removal of the oxide film which is formed upon aluminium when it is in contact with air. Various methods of removing the oxide film to enable aluminium or aluminium alloy parts to be joined together have been employed.
Basically there are three methods of removing the oxide film. The first method is to use a flux which essentially consists of a mixture of chloride and fluoride salts of the alkaline-like metals potassium, sodium, lithium and aluminium. Essentially the flux is maintained at a temperature between the melting point of the brazing alloy and the melting point of the core aluminium and the assembled structure is dipped into the molten flux to melt the brazing alloy and effect the joint.
A second method is to use a furnace brazing operation in which the parts to be joined are heated in a furnace in an atmosphere which is substantially oxygen-free. Various alloys are used in furnace brazing which are intended to enable flow of the brazing alloy to occur and to enable the aluminium parts to be wetted through the oxide layer.
More recently there has been developed a process of brazing aluminium in a vacuum, the process involving the use of a magnesium containing brazing alloy. The magnesium effectively displaces the oxide film and acts as a getter and so enables the aluminium parts to be brazed together. The process of brazing in the presence of magnesium is described in British patent specification No. 1 067 024.
Each of the processes used to join the aluminium structures together have their attendant problems. Thus, with flux brazing the fluxes tend to be corrosive and have to be removed after brazing has occurred. Furnace brazing has problems associated with the certainty of making joints throughout the entire structure, owing to the presence of the oxide film. Vacuum brazing has advantages over both flux dip brazing and furnace brazing except that it involves the use of expensive high vacuum furnaces, but it does enable very high quality products to be prepared. Because of the need to provide a vacuum articles produced by vacuum brazing have, in the main, been relatively small, such as small turbo chargers, radiators for vehicles etc.
It has now unexpectedly been discovered that there are particular problems associated with vacuum brazing large section structures by the route described in the above patent specification. By "large section structures" as is used herein is meant structures which are of a size greater than a cube of 0.25 m length along each side. The problems referred to above basically relate to strength of the brazed joints in the heat exchanger.
The quantity of magnesium in the alloy must be sufficient to permit adequate brazing of the alloy in a vacuum. Typically magnesium contents below about 0.5% would be unlikely to give adequate vacuum brazing of structures, particularly large structures. However, it may be possible for the magnesium to be present in the core of clad alloys.
Literature surveys do not indicate any differences in the strength of structures produced by flux dip brazing, whether such structures are of large section or small section. It would be thought, therefore, that there would be no difference in strength between vacuum brazed structures which are of large or small section.
It has now been discovered that higher strength structures can be provided by utilising the brazing alloys of the present invention in the methods described herein. The strength of the large section structures is particularly noted in relation to the burst pressure at which heat exchangers fail when internally pressurised after brazing in accordance with the prior art and in accordance with the present invention. These failures are not fatigue failures but merely tensile or shear failures resulting from the application of internal pressure.